[1] How many generations can these 2 families live for until there are no more possible unrelated combinations of males and females able to produce?
Three generations: The original four, their kids, and grandkids.

[2] How many people will have been alive after everyone dies out?
The four originals, their 16 children, and the 128 grandkids. Total - 148.

[3] What will have been the most people alive at one time?
148 - the originals being 72, their first children being 54, their second children being 36. The first set of grandkids being 18, and the second set having just been born.

[4] How many years will these people survive for?
In total, from the origainl four at 18, to the grandkids deaths; 127 years.

[EDIT] Not sure about the answers to three and four now.